A number of different types of artificial shoulder joints have been proposed heretofore, and patent and other literature describing such joints will be set forth at a later point in the present specification. However, it appears that for the most part these artificial shoulder joints were based on some preconceived concepts of the designer of the joints and generally did not take into full consideration the exact anatomical configuration of the real human shoulder joint.
From an overall standpoint, the shoulder joint is relatively unconstrained. It includes a matching ball and socket, but the ball and socket members are held in their relative positions by a "rotator cuff" which includes a heavy layer of muscles and ligaments which surround the joint, and which also control the overall movement of the arm relative to the body. Although the shoulder has been referred to casually as a "non-weight-bearing joint," the compressive force acting on the shoulder joint reaches aproximetely 0.8 of body weight when the arm is raised horizontally, medically referred to as 90 degrees abduction, under static conditions. Accordingly, in the course of undertaking heavy work or athletic activities, the shoulder frequently carries loads substantially greater than the human body weight. Certainly, therefore, the shoulder, medically referred to as the glenohumeral articulation, is a major load-bearing joint.
Concerning terminology, the upper arm bone is the humerus, and the ball or rounded joint member at the upper end of the humerus fits into a socket in the shoulder bone, or scapula. the term "glenoid" refers to a pit or a socket, including that in the scapula which receives the ball at the upper end of the humerus. Accordingly, the shoulder joint is sometimes referred to as the glenohumeral articulation.
A number of the prior artificial shoulder joint proposals have involved fully constrained geometries. In such fully constrained geometries, at the limit of travel of the one joint element relative to the other, a positive stop is encountered, and the resultant forces between the arm and the shoulder must necessarily be applied to the joint between the prosthesis and either the humerus and the scapula or both, as the case may be. Unfortunately, the scapula is a relatively light bone, without the depth and massiveness required for very high strength securing of a prosthesis; accordingly, failure rates have approached 50% relative to the glenoid prosthesis in a number of the constrained joint configurations.
In other geometries which have been proposed, the precise shape of the humerus and/or the scapula in human beings has not been closely studied by the designer, so that the resultant joint does not provide a natural movement for the user, or the mode of securing is less than optimum because of the failure to match the prosthesis with the shape of the humerus, or the glenoid recess in the scapula, or both.
Accordingly, a principal object of the present invention is to provide a relatively unconstrained artificial shoulder joint conforming as far as possible to the configuration of the human body, both with regard to the configuration of the joint surfaces, and also with regard to the matching of each prosthesis with the bones to which they must be secured.